The army regulars (mooks) are important to attack-
High level opponents WILL be brought back. No doubt about it. They will be Raised, Ressurected, etc. There's no way the Valloreans will leave that much experience and combat firepower go to waste. Assaults on the high level opponents will generate high-level losses on your side.
Not every mook can have the same done for them, especially if the bodies are taken/mangled/etc. The XP drain on the clerics is too high to be able to do this for everybody.
Morale will begin to falter: "What do you mean High Lord Muckety-muck got brought back from the dead?!??!! I've been losing men all week in these damnable raids! What if I die next? I don't want to die, I won't get the chance to come back! Don't they care about us?"
An army needs infantry to hold ground. If the Valloreans roll up to the gates with only their high-level guys, they may be able to take the fortress, but won't be able to hold the countryside or protect supply lines.
By Agback:
The dictionaries after a time admit common misuses, but there is no reason why you should. In a language with a range of meaning from 'scotch' to 'annihilate', there is no good reason to say 'decimate' when you mean 'kill'.
Sorry, it's the way I've always used it or heard it used.

I actually didn't know until I looked at Webster. Well, learn something new every day...
[poke polite fun with no offense meant ON]
I'll take it for what it's worth to be lectured in the use of acceptable and/or common usage English by someone from a continent where the common greeting is a contraction ("G'day!")

[poke polite fun with no offense meant OFF]